Thread tension control signal output device for sewing machine

ABSTRACT

The invention is to provide an electrical indication of improper thread tension to enable the sewing operation to be exactly accomplished.

BRIEF DESCRIPTION OF THE INVENTION

The invention is to give detecting signals of the thread tension to anelectric circuit in which proper relation in tension between an upperthread and a lower thread is present, or set adjustably, in order toissue output signals indicating proper or improper relationtherebetween, and to maintain an output for automatic adjustment of thethread tension, or point out the need of the manual adjustment.

Generally, some knowledge is required for determining if or not thethread tension is proper. Even those skilled have difficulties indiscerning clearly stitches when the upper and lower are in the samecolor.

The present invention has been devised to eliminate such disadvantagesof the prior art.

A basic object of the invention is to detect the tension of the upperthread and the tension of the lower thread for keeping the tension inproper balance, and compare one thread tension with the other, and, ifthe tension balance has been lost, give a signal to a control circuit orindicate such condition, so that the tension balance can be amended.

A second object of the invention is to provide favorable and attractivestitches through said detection of the unbalance of the thread tensionand by the manual or automatic adjustment of the thread tension.

Many other features or specific effects of the invention will beapparent from the following description of the preferred embodiment ofthe invention in reference to the attaching drawings.

BRIEF DESCRIPTION OF THE INVENTION

FIGS. 1A, 1B and 1C show an installation of the thread tension device ofthe invention,

FIG. 2 shows a control circuit diagram of the above, and

FIGS. 3A and 3B show wave forms of thread tension detecting signals.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the invention illustrated in the drawings isas follows; FIG. 1 shows an installation of a thread tension detectingdevice of the invention, and shows in (B) and (C) side views of theprincipal part of the device viewed from B and C directions. The numeral1 designates a machine housing. The numeral 2 is a thread guide. 3 showsan elastic member for detecting the tension of the upper thread. Upperthread 5 going through a thread tension adjuster 4 is guided through atakeup lever 6, an upper thread hole 7 of a thread guide 2, a threadloop 8 on elastic member 3, and a lower thread hole 9 of the threadguide 2. The thread loop 8 is biased away from the straight lineextending from the thread hole 7 to the other thread hole 9, and whenthe upper thread 5 is pulled during stitching thread loop 8 displaces afree end of the elastic member 3 whose end is fixed by a screw 10extending to the right in FIG. 1(B), although not shown there. G₁ is astrain gauge attached to elastic member 2 and detecting the upper threadtension to and by detecting the degree of distortion of the elasticmember. The numeral 11 is a bobbin carrier of a horizontal race. Thenumeral 12 is an elastic member detecting the lower thread tension. Thenumeral 13 is a feed dog and a lower thread 15 supplied from the bobbin14 is guided to an edge part 16 of the bobbin carrier, and edge part 17of the elastic member 12 and an arm 18 of the feed dog 13, and passesthrough a needle hole 20 of the needle plate 19 after going through aneedle eye (not shown). The edge part 17 is biased to push the lowerthread 15 away from the straight line extending from the edge part 16 tothe arm 18. When the lower thread 15 is withdrawn from the bobbin 14 thethread feed regulating member (not shown) in the bobbin carrier 11tensions the lower thread 15. Pressure proportional to this tension actson the edge part 17, causing friction (proportional in turn, to thispressure) to act on the edge part 17 and to displace the free end 17.The other end of elastic member 12 is fixed with an unshown screw 21which actually extends, towards the right in FIG. 1(C). G₂ is a straingauge attached to elastic member 12 for detecting the lower threadtension by detecting distortion of elastic member 12.

FIG. 2 is a control circuit. The strain gauges G₁ and G₂ are eachconnected as a resistance element in a corresponding bridge having DCpower source E and E' respectively to detect the thread tension of theupper thread and the lower thread.

Resistors R₁, R₂ and R₃ constitute a bridge for detecting the upperthread tension and a variable resistance VR₁ is preset or adjusted insuch a way that when the strain gauge G₁ shows 0 distortion, terminal Uis at nearly ground potential. The electric potential of a terminal Ugenerates a pulse as shown in FIG. 3(A) (the drawing indicates negativeelectric potential) per rotation of the sewing machine due to reductionof the resistance of the strain gauge G₁ caused by increased tension ofthe upper thread during sewing. In FIG. 3A the straight line shows 0electric potential and the time axis. A circuit composed of theoperational amplifier OP₁, resistance R₄, R₅ and R₆ is a first amplifierwhich receives the voltage at terminal U at the inverting input terminalof operational amplifier OP₁ through the capacitor C₁ and the resistanceR₅. The capacitor C₁ is for blocking DC when the adjustment of thevariable resistance VR₁ is insufficient or DC caused by changes in thetemperature in order to automatically adjust and correct distortionwhich will be brought to 0. A circuit composed of the operationalamplifier OP₂, resistances R₇, R₃ and R₉ is a second amplifier whichreceives the output of the operational amplifier OP₁ at inverting inputterminal of operational amplifier OP₂ through a resistance R₇ forinverted amplification. A low-pass filter circuit comprising anoperational amplifier OP₃, resistances R₁₀, R₁₁ and capacitors C₂ and C₃blocks high frequency inductive components caused by outside noises, andits output is inverted. Operational amplifiers OP₄, and OP₅, resistancesR₁₃, R₁₄ and R₁₅, variable resistances VR₂ and capacitors C₄, C₅ and C₆all form a notch filter to cut off induction from commercial AC powersources, and this circuit receives the output from the operationalamplifier OP₃ and gives outputs from the operational amplifier OP₄. Thevariable resistance VR₂ is for tuning and adjusts the cut off frequencyof the notch filter. A peak holding circuit comprising an operationalamplifier OP₆ and OP₇, a diode D, a resistance R₁₆ and a capacitor C₇charges capacitor C₇ and holds the voltage across it until the voltageoutput from the operational amplifier OP₆ reaches the peak value of thevoltage whose wave form is nearly equal to that illustrated in FIG. 3(A)and causes the operational amplifier OP₇ to output the voltage so held.The structure of the control circuit including low thread tensiondetecting strain gauge G₂, and the function of individual elementstherein are almost identical to those of the control circuit includingstrain gauge G₁, and corresponding functional elements are primed. Theelectric potential at terminal U' generates an identical pulse duringsewing, caused by reduced resistance of the strain gauge G₂ due to theincreased tension of the low thread 15, which pulse is approximately180° out of phase with the corresponding pulse from the strain gauge G₁,as is shown in FIG. 3(B). The amplification is varied with the variableresistance VR.sub. 3 having an adjustment part outside of the sewingmachine. Based on adjustment of variable resistance VR₃, the outputsfrom operational amplifiers OP₇ and OP'₇ set sensitivity of the straingauges G₁ and G₂ be nearly equal adjustment value when the tensionbalance between the upper thread and the lower thread is proper. Theoperational amplifier OP₈ is a first comparator which receives theoutput of the operational amplifier OP₇ at the inverting input terminalof operational amplifier OP₈ via a resistance R₁₇, and receives theoutput of the operational amplifier OP'₇ at its non-inverting inputterminal of operational amplifier OP₈ a resistance R₁₈, Zener diode ZD,and resistance R₁₉. R₂₀ is a voltage-divided resistance, and R₂₁ is afeedback resistance, and the operational amplifier OP₈ inverts andamplifies the output of the operational amplifier OP₇. Tr₁ is a firsttransistor which receives at its base the output of the operationalamplifier OP₈ via resistance R₂₂, and the collector of first transistorTr₁ is connected with a cathode of a luminous diode LED₁ (for indicatingexcessive lower thread tension) which receives the power source (+V).R₂₃ is a base-emitter resistance, and R₂₄, is a current limitingresistance. The operational amplifier OP'₈ is a second comparator whichreceives the output from operational amplifier OP₇ at its non-invertinginput terminal of operational amplifier OP'₈ via Zener diode ZD', and aresistance R'₁₉. Tr₂ is a second transistor which receives at its basethe output of an operational amplifier OP'₈ via the resistance R'₂₂ andthe collector is connected with the cathode of a luminous diode LED'₂which indicates excessive upper thread tension. The Zener diodes ZD, ZD'are provided with characteristics such that a corresponding luminousdiode LED₁ or LED.sub. 2 lights only when the lower thread tension orthe upper thread tension is higher than a fixed standard to preventdiodes LED₁ and LED₂ from always lighting alternately. The bangedswitches SW₁ and SW₂ are operable outside of the sewing machine, or areoperated by selection of patterns, in order to switch tension balancebetween the straight stitching and the other pattern stitching. Whenreleased, switches SW₁ and SW₂ prompt lighting of the excessive upperthread tension indicator LED₂, and when closed, they prompt lighting ofthe excessive lower thread tension indicator LED₁. In the patternstitching, it is desirable to have crossing point between the upperthread and the lower thread at a point slightly below the center of thecloth. For this reason, it is desirable to use the switch to reduce theupper thread tension from the tension used in straight stitching. S₁,S₂, S'₁ and S'₂ are output terminals which are connected to an automaticadjustment control circuit of the upper thread and the lower thread (orthe upper thread only) which is independently provided with an upperthread tension peak hold signal, the excessive lower thread tensionsignal, the lower thread tension peak hold signal and the upper threadtension signal. When the straight stitch is selected in the abovestructure, the switches SW₁, SW₂ are closed. The variable resistance VR₃is set to the standard value. Every time the take-up lever rises on eachrotation of the sewing machine, the upper thread 5 is pulled and theresistance of the strain gauge G₁ for detecting the upper thread tensionis reduced along the curve of the elastic member 3, and a negative pulseis generated at the terminal U as shown in FIG. 3(A). This pulse isamplified via operational amplifiers OP₁ to OP₄ and OP₆, and a peakvalue of an exactly inverted version of this pulse is held by thecapacitor C₇. The output which is voltage-followed by the operationalamplifier OP₇ is given to the inverting input terminal (-) of theoperational amplifier OP₈, and is also given to the non inverting inputterminal (+) of the operational amplifier OP'₈. The elastic member 12 isdeformed as the lower thread is supplied from the bobbin 14 and thestrain gauge G₂ detecting the lower thread tension is reduced in theresistance and the terminal U' is generated with negative pulse shown inFIG. 3(B) whose phase is shifted 180° from that shown in FIG. 3(A). Thispulse is amplified via the capacitor C'₁, and the operational amplifiersOP'₁, OP'₂, OP'₃, OP'₄ and OP'₆, and the peak value an inverted versionof this pulse is held at the capacitor C'₇. The voltage-followed outputfrom operational amplifier OP'₇ is routed to the non inverting inputterminal (+) of the operational amplifier OP₈ via the switch SW₁, and isalso routed to the inverting input terminal (-) of the operationalamplifier OP'₈ via the switch SW₂. If the electric potential of the noninverting input terminal (+) of the operational amplifier OP₈ (which isthe first comparator) is higher than that of its inverting inputterminal, it denotes that the lower thread tension is excessively higherthan tension of the upper thread. This causes the output of theoperational amplifier OP₈ to be positive, thereby turning on transistorTr₁ and causing luminous diode LED₁ to light and thus indicate excessivelower thread tension. Normally, in such a case, the holding pressure ofthe thread adjuster 4 is increased by the manual adjustment. At thistime, the electric potential of the non inverting input terminal (+) ofthe operational amplifier OP'₈ (which is the second compactor) is lowerthan the electric potential of its inverting input terminal (-). Theoutput of the operational amplifier OP'₈ is 0, transistor Tr₂ remainsturned off, and the luminous diode LED₂ does not light. If the electricpotential of the non inverting input terminal (+) of the operationalamplifier OP'₈ is higher than that of its inverting input terminal (-),it denotes that the upper thread tension is higher than the lower threadtension, and the output of the operational amplifier OP'₈ turnspositive, causing the luminous diode LED₂ to light, and reducing theholding pressure of the thread tension adjuster 4. When stitchesincluding the needle swinging amplitude such as the pattern stitches areselected, the switches SW₁ and SW₂ are opened, and the resistances R₁₈and R₁₈ ' are placed in circuit. Therefore, the excessive upper threadtension indicating luminous diode LED₂ lights as the thread tensionremains at its previous value (neither of luminous diodes LED₁ and LED₂lights in the straight stitch) and an order is given either to reducethe upper thread tension or to increase the lower thread tension. Thelower thread tension shown in FIG. 3(B) generally remains almostconstant despite changes in the speed of the sewing machine, but theupper thread tension shown in FIG. 3(A) is influenced by the movingcharacteristic of the takeup lever 6 and its wave value increases as thespeed of the sewing machine increases. Even as speed increases, properthread tension is provided by means of the automatic adjustment controlcircuits of the upper and lower threads (the upper thread only) whichare independently provided by utilizing individual signals outputs S₁,S₂, S'₁ and S'₂.

We claim:
 1. An electronic thread tension monitor for monitoring upperthread tension and lower thread tension, comprising:an upper threadtension sensor responding to upper thread tension and generating anupper thread tension signal which is proportional thereto; a lowerthread tension sensor responding to lower thread tension and generatinga lower thread tension signal which is proportional thereto; a firstmemory connected to the upper thread tension sensor and registering peakvalues of the upper thread tension signal; a second memory connected tothe lower thread tension sensor and registering peak values of the lowerthread tension signal; and a comparator stage connected to the firstmemory and the second memory and performing a comparison between peakvalues stored in the first memory and peak values stored in the secondmemory and generating a tension balance signal as a result of thecomparison.
 2. The electronic thread tension monitor defined by claim 1,wherein at least a one of the upper thread tension sensor and the lowerthread tension sensor includes an elastic member bearing against athread and being distorted as a result of tension thereof, and wherein astrain gauge is secured to the elastic member to measure suchdistortion.
 3. The electronic thread tension monitor defined by claim 2,wherein the strain gauge has a strain-variable resistance and isconnected in a resistance bridge.
 4. The electronic thread tensionmonitor defined by claim 1, further including a first low-pass filterconnected between the upper thread tension sensor and the first memoryand a second low-pass filter connected between the lower thread tensionsensor and the second memory.
 5. The electronic thread tension monitordefined by claim 4, further including a first notch filter connectedbetween the first low-pass filter and the first memory and a secondnotch filter connected between the second low-pass filter and the secondmemory.
 6. The electronic thread tension monitor defined by claim 1,wherein at least one of the first memory and the second memory includesa capacitor which is charged by a corresponding one of the upper threadtension signal and the lower thread tension signal through a diode.